Piston type small discharge pump

ABSTRACT

The present invention provides a piston type small discharge pump capable of reducing a dead volume and effects of unbalanced load to the piston shaft, and being self-priming. The fluid taken through a inlet port is introduced into a cylinder portion via a flat type check valve, and discharged into a discharge port through a discharge hole and a check valve as a piston moves up and down. A flat type check valve is provided in the vicinity of a bottom dead center of the piston and a spring for biasing the piston upwardly is provided outside the cylinder portion, thus reducing the dead volume.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a piston type pump such as a piston type small discharge pump capable of discharging fluid by a small discharge of about 10 cc/minute, for example.

[0002] Conventionally many types of pumps have been generally used. For pumps for discharging a small amount, piston type of pumps are in the majority. In order to discharge a small amount, a discharge amount per unit time is reduced by means of a drive control mechanism for driving a piston by extending time per stroke. However, the discharge amount is too large to provide such a small discharge accurately.

[0003] In other words, most of conventional type pumps convert revolving motion of a rotating shaft to reciprocating motion by means of a eccentric cam mounted on the rotary shaft so as to reciprocate a piston. Driving the piston by means of the eccentric cam is carried out by a mechanism where the piston is brought in direct contact with the eccentric cam. Therefore turning force of the eccentric cam is applied to the piston in addition to translational driving power, thereby causing an earlier abrasion and resulting in inaccurate reciprocating motion and fluctuant discharge.

[0004] Most springs for reliably bringing the piston in contact with the cam are inserted inside a cylinder. Since there is the spring, a pump chamber volume at the bottom dead center of a discharging step, that is a dead volume, cannot be small. The dead volume significantly affects self-priming of the pump.

[0005] Now, the self-priming of the pump will be discussed. In a piston type pump, when a pump chamber is filled with air, pumping only the air results in that only the air is compressed and expanded by the reciprocating motion, which may have a risk of not discharging fluid.

[0006] If the pump is self-priming, even when the pump chamber has no fluid, the pump may be driven to discharge the air, thereby pumping fluid later. In a piston type pump, if a dead volume which is a volume of a pump chamber at a bottom dead center of a discharging step is zero, then air is taken in during an inlet step after the bottom dead center and the taken air is completely discharged during a discharging step. Repeating these steps, the air is discharged and fluid is reliably taken into the pump chamber.

[0007] However, if there is a little dead volume where some air exists, air is not taken in during an inlet step until an inner pressure reaches a threshold, because air may be compressed and expanded. Further, during the discharge step, air is not discharged either until the inner pressure reaches the threshold.

[0008] Therefore, if the piston cannot move beyond the threshold, the pump is not self-priming. Even if the piston may move beyond the threshold, the amount of air discharged by one cycle of inlet step and discharge step is small compared with the maximum volume of the pump chamber. Thus, in order to increase its self-priming, it is necessary to make the dead volume as small as possible.

[0009] In a piston type pump, it is possible to adopt a layout where the dead volume at the bottom dead center is reduced. On the other hand, in a diaphragm type pump where a spring is set only inside, a dead volume cannot basically be zero. Therefore, in view of providing self-priming, the piston type pump more advantageous than the diaphragm type pump. A small discharge pump, however, cannot provide a large volume of a pump chamber at the top dead center and therefore it is difficult to provide self-priming.

[0010] Further, check valves are necessary at an inlet port and a discharge port in order to do pumping by means of reciprocating motion of a piston, however, in particular, the check valve at the inlet port mostly affects the dead volume. In a small discharge pump, there are problems in its dead volume and in that a spring constant of its check valve.

SUMMARY OF THE INVENTION

[0011] An object of the invention is to provide a piston type small discharge pump allowing a small dead volume, reducing effects of unbalanced load to a piston shaft, and providing self-priming.

[0012] Another object of the present invention is to provide a piston type small discharge pump characterized in that a spring for biasing a piston towards a top dead center is provided outside a cylinder.

[0013] Another object of the present invention is to provide a piston type small discharge pump characterized in that a flat type check valve is disposed in the vicinity of a bottom dead center of a piston so as to reduce a dead volume at the bottom dead center.

[0014] Another object of the present invention is to provide a piston type small discharge pump characterized in that there are provided an internal recycling passage where an outlet side of a check valve disposed at an outlet side of a cylinder is communicated with an inlet side of a check valve at an inlet port side of the cylinder, and a flow rate adjusting valve in the internal recycling passage.

[0015] Another object of the present invention is to provide a piston type small discharge pump characterized in that there is provided a flow rate adjusting valve at a discharge port side.

[0016] Another object of the present invention is to provide a piston type small discharge pump characterized in that there is provided a check valve at the internal recycling passage.

[0017] Another object of the present invention is to provide a piston type small discharge pump characterized in that there is provided either or both an outer diameter reduced portion where the outer diameter is reduced at a lower portion of a piston and an inner diameter increased portion where the inner diameter is increased at a lower portion of a cylinder.

[0018] Another object of the present invention is to provide a piston type small discharge pump characterized in that an envelope of a cam portion, mounted in the middle of a rotary shaft, for driving a piston is defined not to extend beyond an outer shape of the rotary shaft.

[0019] Another object of the present invention is to provide a piston type small discharge pump characterized in that an intermediate member which substantially does not move as the cam portion rotates is interposed between the cam portion and a top of the piston driven by the cam portion.

[0020] Another object of the present invention is to provide a piston type small discharge pump characterized in that the cam portion is column-shaped and eccentric of the central axis of the rotary shaft.

BRIEF DESCRIPTION OF DRAWINGS

[0021]FIG. 1 is a partially sectional view for explaining the first embodiment of a piston type small discharge pump according to the present invention.

[0022]FIG. 2 is a schematic illustration for explaining relationship between a cam member and an intermediate lever.

[0023]FIG. 3 is a schematic illustration for explaining relationship between the cam member and an intermediate member.

[0024]FIGS. 4A and 4B are schematic illustrations for explaining the cam member.

[0025]FIG. 5 is a schematic illustration for explaining a flat valve.

[0026]FIGS. 6A and 6B are schematic illustrations for explaining an example of a check valve.

[0027]FIG. 7 is a schematic diagram showing a piston type small discharge pump according to the first embodiment.

[0028]FIG. 8 is a partially sectional view for explaining the second embodiment of a piston type small discharge pump according to the present invention.

[0029]FIG. 9 is a schematic diagram showing a piston type small discharge pump according to the second embodiment.

[0030]FIG. 10 is a schematic representation for explaining the third embodiment of a piston type small discharge pump according to the present invention.

[0031]FIG. 11 is a schematic diagram showing a piston type small discharge pump according to the third embodiment.

[0032]FIG. 12 is a partially sectional view for explaining the third embodiment of a piston type small discharge pump according to the present invention.

[0033]FIG. 13 is a schematic illustration of an inlet port.

[0034]FIGS. 14A and 14B are schematic sectional views for explaining an example of the discharge adjusting valve as shown in FIG. 12.

PREFERRED EMBODIMENT OF THE INVENTION

[0035]FIG. 1 is a partially sectional view for explaining the first embodiment of a piston type small discharge pump according to the present invention.

[0036] With respect to the view, reference numeral 1 depicts a base, reference numeral 2 shows a piston, reference numeral 3 is a cylinder portion, reference numeral 3 a is a enlarged portion, reference numeral 4 depicts a spring, reference numeral 5 shows a holding plate, reference numeral 6 is a washer, reference numeral 7 depicts a snap ring, reference numeral 8 shows an O-ring, reference numeral 9 is a rotary shaft, reference numeral 9 a depicts a central point, reference numeral 10 shows a gear, reference numeral 11 is an eccentric portion, reference numeral 11 a is a central portion, reference numeral 12 depicts an intermediate lever, reference numeral 13 shows an oil retaining metal, reference numerals 14 and 15 are snap rings, reference numeral 16 depicts a check valve, reference numeral 17 shows a holding member, reference numeral 17 a is an inlet hole, reference numeral 18 depicts a stopper member, reference numeral 18 a is an inlet hole, reference numeral 19 is a discharge hole, reference numeral 20 depicts a check valve, reference numeral 21 shows a holding member, reference numeral 22 is an inlet port, and reference numeral 23 is a discharge port.

[0037] The piston 2 is slidably inserted into the cylinder portion 3 which is provided as a circular hole at the base 1 and the piston 2 is biased upwardly by means of the spring 4. The spring 4 is held at its upper portion by the holding plate 5; the lower portion of the spring 4 is provided on the washer 6; and the holding plate 5 is retained by the snap ring 7. As such the spring 4 is provided not inside but outside the cylinder portion 3 and therefore its dead volume may be minimized.

[0038] O-rings 8 for sealing are double-decked below the portion where the spring 4 is provided. One O-ring 8 may be sufficient, however, it is preferable to stack a plurality of O-rings 8 in order to prevent a very small leak, thus two O-rings are applied to this embodiment. More than 2 decks may be available, and a multiple-deck of O-rings may prevent a leak almost completely. Since a stroke of the piston 2 is small (2 mm, for example), the O-ring 8 may be of synthetic rubber.

[0039] The piston 2 biased upwardly by the spring 4 is driven by means of the rotary shaft 9. The gear 10 is attached to the rotary shaft 9. Turning force is transmitted through a gear (not shown) which engages with the gear 10, whereby the rotary shaft 9 is rotated. The eccentric portion 11 is provided as the cam portion at a portion of the rotary shaft 9, and a top face of the piston 2 is abutted contact with the eccentric portion 11 via the intermediate lever 12.

[0040] The rotary shaft 9 is inserted into a bearing hole formed of the oil retaining metal 13 which is formed at a flange mounted on an upper portion of the base 1. Further, snap rings 14 and 15 are attached to the rotary shaft 9 so as to prevent removal.

[0041]FIG. 2 is a schematic illustration showing relationship among these elements, and the rotary shaft 9 rotates about a central axis through the central point 9 a. The eccentric portion 11 formed at the intermediate portion of the rotary shaft 9 has a circular cross section where the central point 11 a is eccentrically provided to the central point 9 a of the rotary shaft 9. As the rotary shaft 9 rotates, the top face of the piston 2 biased upwardly moves up and down so as to always follow the eccentric portion 11.

[0042] The intermediate lever 12 which substantially does not move as the cam portion rotates is interposed between the eccentric portion 11 and the top face of the piston 2, so that the top face of the piston 2 is not rubbed with the eccentric portion 11, thereby preventing from causing abrasion of the top face of the piston 2, increasing its durability, and reducing effects of unbalanced load to the piston 2.

[0043] The intermediate lever 12 may be of a leaf spring made of metal such as phosphor bronze, and may be of synthetic resin such as high density polyethylene (HDPE). Further, its base end may be fixed or a metal plate or a synthetic resin plate having no spring action may be flexibly supported at its base end.

[0044] Note that the outer shape of the eccentric portion 11 provided in the middle of the rotary shaft 9 is preferably formed not to extend beyond the outer shape of the rotary shaft 9. As such the rotary shaft 9 is not bothered when inserted into a bearing or the oil retaining metal 13 as in the above example.

[0045] The method of driving the piston 2 by means of the rotary shaft 9 is not limited to interposing the intermediate lever 12, and may be provided with a column-shaped intermediate member 12 a as shown in FIG. 3. In summary, it merely requires an intermediate member which substantially does not move as a cam member moves such that the top face of the piston 2 is not rubbed by the eccentric portion 11 which functions as the cam member, and therefore the shape and the support mechanism of the intermediate member is optional. As such the durability of the piston 2 may be improved.

[0046] In this embodiment, the eccentric portion 11 of eccentrically circular shape is provided at a portion of the rotary shaft 9 as the cam member, however, the cam member is not limited to the eccentrically circular shape.

[0047] A cam capable of providing reciprocating motion and having a suitable shape may be used. For example, a cam member 11 b of an ellipse shape as shown in FIG. 4A and a cam member 11 c of a triangle shape having somewhat round edges as shown in FIG. 4B may be used. The envelope of the cam portion preferably does not extend beyond the outer shape of the rotary shaft 9. Further, not only revolving drive but also reciprocating drive or the like may be used as a suitable driving means.

[0048] Now, referring back to FIG. 1, in a situation where the piston 2 is as low as possible, that is, when the piston 2 is at the bottom dead center, a check valve 16 is disposed in the vicinity of the bottom face of the piston 2. The check valve 16 is constructed as a flat valve. As shown in FIG. 5, a central portion is defined by a cut-out portion 16 b so as to be a valve portion 16 a. Due to elasticity of a portion where the cut-off portion 16 b is not formed, that is, a portion where the valve portion 16 a communicates with a peripheral portion 16 c, restoring force is provided, and the check valve 16 may be formed of a metal thin plate such as a thin plate of phosphor bronze or a thin plate of a synthetic resin such as polyethylene.

[0049] The valve portion 16 a is disposed so as to close the inlet hole 17 a and secured by the holding member 17. The holding member 17 is secured to a hole formed at the base 1 by a suitable means such as mated engagement, screw engagement, or the like.

[0050] The inlet hole 17 a is formed at the holding member 17 and communicated with the inlet port 22 via the cut-off portion 18 a formed at the stopper member 18. The stopper member 18 is secured so as to make screw engagement with a hole formed at the base 1 and the holding member 17 may be secured by the stopper member 18.

[0051] Note that the check valve 16 is not limited to the structure as shown in FIG. 5. Check valves using a ball valve biased by a spring and having other structure may be used. In order to minimize the dead volume as small as possible, the flat type check valve as shown in FIG. 5 is more advantageous.

[0052] The discharge hole 19 is provided somewhat above the check valve 16. The enlarged portion 3 a is formed so as to overlap at least a portion of the discharge hole 19 and so that the lower portion of the cylinder 3 is conically enlarged. When the piston 2 gets closer to the bottom dead center, fluid may flow out into the discharge hole 19 through a gap provided by the enlarged portion 3 a. The check valve 20 is disposed on an outlet side of the discharge hole 19. The check valve 20 is attached by the holding member 21 which is secured in screw engagement with the hole formed at the base 1. In this embodiment, the holding member 21 is screwed into a threaded hole formed at the base 1, so that the pressure of the check valve 20 may be adjusted by adjusting the screw engagement. The holding member 21 may be press fit so as to secure, so that the pressure of the check valve 20 may be avoided. The holding member 21 has a hole formed to let fluid flow and is communicated with the discharge port 23.

[0053] An example of the check valve 20 is shown in FIGS. 6A and 6B. FIG. 6A shows the check valve 20 in a flat plate state. FIG. 6B is a perspective view thereof. Referring to the FIGS., reference numerals 20 a and 20 b respectively depict a valve portion and a spring portion.

[0054] A metal plate of a spring material is punched out by means of press working such that the valve portion 20 a is formed in the center portion, subsequently the spring portion 20 b is formed so as to provide spring force to the valve portion 20 a as shown in FIG. 6A, and formed as shown in FIG. 6B, whereby a flat valve and a spring are integrally formed with the same material so as to be used as a check valve.

[0055] Since the valve portion 20 a and the spring portion 20 b are integral, it is not necessary to consider about centering, and a desired spring constant may be obtained by selecting the thickness and the material.

[0056] Instead of the spring portion 20 a as shown in FIGS. 6A and 6B, a conventional coil spring or cone spring could be used by welding or soldering a flat valve thereto so as to be a check valve having similar function, however, in practice it is difficult to obtain sufficient parallelism and centering, thereby being costly. In consideration of reliable function, simple structure, cost and the like, it has been found that the method of punching a flat plate by means of press working is optimal.

[0057] The check valve 20 is not limited to the structure as described with reference to FIGS. 6A and 6B. Check valves using a ball valve biased by a spring and those having other structure may be used.

[0058] In the piston type small discharge pump having this structure, the piston 2 repeats up and down motion by the revolving force transmitted to the gear 10 so that the fluid taken through the inlet port 22 may be discharged into the discharge port 23. Metals and synthetic resins may be used as the material of the base 1. The cylinder 3, an insertion hole of the O-ring 8, an accommodating hole of the spring 4, a mounting hole of the check valve 16, a mounting hole of the holding member 21, an inserting hole of the holding member 17, the discharge hole 19, a mounting hole of the check valve 20, an inserting hole of the holding member 21, and the like may be formed of a hole having a circular section, so that mounting holes and the like for the elements may be formed at a block of the base 1 by means of rotary cutting tools, thereby providing an advantage in easier manufacture.

[0059] When starting to use the pump, if there is no fluid around the lower portion of the piston 2, the piston 2 moves up and down, takes air in, discharge the air out of the discharge port 23, and repeats these actions so as to take in the fluid through the inlet 22. In this case, since the dead volume is small, it does not take a long period of time to take the fluid in and to discharge into the discharge port 23.

[0060] The basic structure of the piston type small discharge pump as described in the embodiment is shown in FIG. 7. In FIG. 7, portions corresponding to those as in FIG. 1 have the same reference numerals and descriptions of them are omitted. The fluid through the inlet port 22 is taken by the piston 2 and the cylinder portion 3 via the check valve 16 and is discharged into the discharge port 23 via the check valve 20. The amount of discharge per minute is determined according to the product of the discharge volume caused by one stroke of up and down motion and the number of times of up and down motion of the piston per minute.

[0061] As such a small flow rate pump may have a piston of a small diameter and of a short stroke so as to obtain a small discharge per stroke. In such a small controllable volume, pumping may not be carried out due to contamination of a very small amount of air. The reason is that the air becomes compliant so that reliable inlet cannot be performed.

[0062] As described above, this embodiment has advantages in that the dead volume may be minimized by disposing the spring 4 of the piston 2 not inside but outside the cylinder portion 3, and disposing the check valve 16 of the inlet side in the vicinity of the bottom dead center of the piston 2. If some air is contaminated, the air is easily discharged and pumping is carried out.

[0063] Further, whether or not easy inlet is performed depends on the force to move inlet valves, so that a smaller force to control inlet valves is preferable.

[0064]FIG. 8 is a partially sectional view for explaining the second embodiment of a piston type small discharge pump according to the present invention. In FIG. 8, portions corresponding to those as in FIG. 1 have the same reference numerals and descriptions of them are omitted. Reference numeral 24 depicts a recycling passage, reference numeral 25 shows a recycling amount adjusting valve, reference numeral 26 is a discharge adjusting valve, reference numeral 26 a is a cut-off portion, reference numeral 27 depicts a valve seat, and reference numeral 28 shows packing.

[0065] In this embodiment, the piston type small discharge pump according to the first embodiment which has been described with reference to FIGS. 1 to 7 is modified to provide the recycling passage 24 so that the outlet side of the check valve 20 is communicated with the inlet side (the inlet hole 17 a side) of the check valve 16.

[0066] Further, in order to adjust a circulation area of the recycling passage 24, the recycling amount adjusting valve 25 is provided. The recycling amount adjusting valve 25 is screwed into the threaded hole formed at the base 1 and its distal end of conical portion extends into the recycling passage 24 passing through a hole formed at the stopper member 18. As the recycling amount adjusting valve 25 rotates, the distal end of conical portion advances and retracts so as to adjust the circulating area. Note that the packing 28 is of an elastomer such as rubber and provided to prevent a leak.

[0067] Further, the discharge adjusting valve 26 is provided on the discharge port 23 side. In this embodiment, the discharge adjusting valve 26 is made in screw engagement with a female screw threaded inside a pipe which forms the discharge port 23, and mounted so as to advance or retract by rotation from the outlet side.

[0068] The distal end of conical portion of the discharge adjusting valve 26 extends into the valve seat 27 secured at a hole formed at the holding portion 21, a small gap between the distal end of conical portion and the valve seat 27 forms a discharge passage for the fluid. Note that the cut-off portion 26 a is formed in the axial direction at a part of peripheral area where male screw of the discharge adjusting valve 26 is formed, and spaces in front of and behind the discharge adjusting valve 26 are communicated with each other so as to form a discharge passage.

[0069] The discharge adjusting valve 26 is not adjustable in a situation where piping is provided at the discharge port 23. Generally, it is adjusted when shipping. The discharge adjusting valve according to this embodiment is not limited to the above. A suitable flow rate adjusting valve is provided at the passage between the check valve 20 and the discharge port 23. Of course, the discharge adjusting valve is provided so as to be externally adjustable like the discharge adjusting valve 26 as in FIG. 8 because it is not limited to a discharge adjusting valve incapable of externally adjusting.

[0070] In this embodiment, a discharge less than that made by a full-stroke of the piston 2 may be discharged from the discharge port 23. In other words, the recycling amount adjusting valve 25 and the discharge adjusting valve 26 are adjusted such that most of the discharge by a full-stroke, 90% for example, is recycled through the recycling passage 24, the rest 10% is discharged from the discharge port 23.

[0071] For example, assuming that a pump discharges by 10 cc per minute, the piston 2 may discharge 100 cc per minute at its full-stroke without internal recycling, when 90% thereof is recycled, the pump is of 10 cc per minute discharge. Further, when a cycle of the piston 2 is constant, that is, a rotating speed of the gear 10 is constant, the discharge may be adjusted over a wide range by adjusting the recycling amount adjusting valve 25 and the discharge adjusting valve 26 so as to vary a recycling rate of recycled fluid into the recycling passage 24. If the rate variation and changing the cycle of the piston 2 are combined, the discharge may be adjusted over a wider range.

[0072] Note that, in this embodiment, the inlet port 22 is communicated with the discharge port 23 via the recycling passage 24. Therefore, when there is no fluid around the lower portion of the piston 2, in operation the pump may not seem to take in air from the discharge port 23 and be self-priming. However, the pump according to the present invention allows a small discharge, the passage of the discharge adjusting valve 26 is very narrow. By balancing its fluid resistance and that of the inlet hole 17 a, the pump may take air from the inlet port 22 side and carry out self-containing function. Further, the pump may be used such that a fluid level on the inlet port side is almost the same as that of the lower end of the piston 2, which is an advantageous use for this embodiment.

[0073] A basic structure of the piston type small discharge pump as described in this embodiment is shown in FIG. 9. In FIG. 9, portions corresponding to those as in FIG. 8 have the same reference numerals and descriptions of them are omitted. The fluid through the inlet port 22 is taken by the piston 2 and the cylinder portion 3 via the check valve 16, and discharged into the discharge port 23 via the check valve 20. The discharge adjusting valve 26 is provided on the discharge port 23 side. The recycling passage 24 is connected between the check valve 20 and the discharge adjusting valve 26 and connected to the check valve 16 at its inlet side. The recycling amount adjusting valve 25 is provided at the recycling passage 24. Therefore, a part of the fluid discharged by the piston 2 and the cylinder portion 3 is recycled to the inlet side of the piston 2 and the cylinder portion 3 via the recycling passage 24. A difference between the discharge through the piston 2 and the cylinder portion 3 and the internally recycled amount through the recycling passage 24 is the discharge into the discharge port 23.

[0074]FIG. 10 is a schematic representation for explaining the third embodiment of a piston type small discharge pump according to the present invention. FIG. 11 is a schematic diagram for explaining a basic structure. In the FIGS. 10 and 11, portions corresponding to those as in FIGS. 1 and 8 have the same reference numerals and descriptions of them are omitted. Reference numeral 29 depicts a check valve.

[0075] In this embodiment, the recycling passage 24 is basically provided similar to the second embodiment, and the check valve 29 is interposed at the recycling passage 24. The interposed check valve 29 prevents the discharge port 23 from communicating with the inlet port 22.

[0076] As shown in FIG. 10, the piston 2 has its lower portion smaller in outer diameter so as to form a passage where fluid flows when the piston 2 comes down. The smaller outer diameter is obtained by forming a stair-shape, however, it may be formed in conical trapezoid having a taper so as to be smaller. The smaller outer diameter of the lower portion of the piston 2 provides similar effects relating to the conically enlarged portion 3 a which is formed by enlarging the lower portion of the cylinder portion 3 into a conical shape.

[0077] Therefore, in this embodiment, instead of making the outer diameter of the lower portion of the piston 2 smaller, an enlarged portion may be formed at a lower portion of the cylinder portion 3 similar to FIGS. 1 and 8. Further, in FIGS. 1 and 8, instead of forming the enlarged portion at the lower portion of the cylinder portion, the outer diameter of the lower portion of the piston may be made smaller. Of course, both the smaller outer diameter of the lower portion of the piston and the enlarged portion of the lower portion of the cylinder portion may be formed.

[0078]FIG. 12 is a partially sectional view for explaining the third embodiment of a piston type small discharge pump according to the present invention. In FIG. 12, portions corresponding to those as in FIGS. 1, 8, 10, and 11 have the same reference numerals and descriptions of them are omitted. Reference numerals 30 and 31 depict check valve mounting holes, reference numerals 32 and 33 are stopper members, and reference numeral 34 is an inlet port.

[0079] Also in this embodiment, metals and synthetic resins may be used as the material of the base 1. The cylinder 3, an insertion hole of the O-ring 8, an accommodating hole of the spring 4, the mounting hole 30 of the check valve 16, the discharge hole 19, a mounting hole of the check valve 20, the recycling passage 24, a mounting hole 31 of the check valve 29, the inlet passage 34, and the like may be formed of a hole having a circular section, so that mounting holes and the like for the elements may be formed at a block of the base 1 by means of rotary cutting tools, thereby providing an advantage in easier manufacture.

[0080] Note that check valves are attached at the mounting hole 30 of the check valve 16 and the mounting hole 31 of the check valve 29, subsequently the stopper members 32 and 33 are respectively screwed into the mounting holes 30 and 31 so as to be air tight.

[0081] A passage between the inlet port 22 and the discharge port 23 will be described. The inlet port 22 is formed of an aluminum alloy, press fit into a hole formed at the base 1, and secured. Of course, other materials may be used. The securing method may be not only press fit but also screwing, soldering or other suitable methods. The inlet passage 34 is formed so as to communicate with the inlet port 22. Assuming that the direction perpendicular to the page where FIG. 12 is shown is the upward direction, the inlet passage 34 is formed below the check valve mounting hole 31, and not communicated with the check valve mounting hole 31. In other words, the inlet passage 34 crosses the check valve mounting hole 31 in cubic interchange so as to be separated above and below.

[0082] The other end of the inlet passage 34 communicates with the check valve mounting hole 30. In the drawing, it seems that the check valve mounting hole 31 and the check valve mounting hole 30 are formed at the same level, however, in fact the check valve mounting hole 30 is formed at a lower level than the check valve mounting hole 31. Therefore, fluid goes through the inlet port 33, the inlet passage 34, the check valve 16, and the inlet hole 17 a to the lower portion of the cylinder portion 3. As the piston 2 moves up and down, the fluid is discharged into the discharge hole 19 passing through the smaller outer diameter portion of the lower portion of the piston 2, and discharged into the discharge port 23 through the check valve 20. The discharge adjusting valve 26 which is provided at a part of the discharge port 23 may adjust the flow rate.

[0083]FIG. 13 is a perspective view looked through the face where the check valve mounting hole appears. The inlet port 22 communicates with the inlet passage 34 which communicates with the check valve mounting hole 30. The check valve mounting hole 31 is formed so as to cross the inlet passage 34 in cubic interchange and communicated with the recycling passage 24. The recycling passage 24 is shown to have the recycling amount adjusting valve 25.

[0084] Referring back to FIG. 12, a part of the fluid passed through the check valve 20 diverges into the recycling passage 24, and flows into the check valve mounting hole 31 via the check valve 29 provided at a part of the recycling passage 24. The check valve mounting hole 31 forms a part of the recycling passage 24. As can be seen from FIG. 13, the check valve mounting hole 31 communicates with the check valve mounting hole 30 via the recycling amount adjusting valve 25, and a part of the fluid discharged from the piston 2 and the cylinder portion 3 is internally recycled. Effects of the internal recycling are similar to the above description of the second embodiment.

[0085] Note that, in this embodiment a cap-shaped cylindrical member 12 b whose top is closed is provided as an intermediate member which contacts with a top of the piston 2 so as to be provided between the top and a pressure member which presses the piston 2. It is preferable to form an air hole 12 c at the top of the cylindrical member. However, the intermediate member is not limited to the cylindrical member 12 b whose top is closed, and may naturally be the intermediate lever as described with reference to FIG. 2, the column-shaped member as described with reference to FIG. 3.

[0086]FIGS. 14A and 14B are shown to explain an example of the discharge adjusting valve 26 as in FIG. 12; FIG. 14A shows a situation where the passage is limited; and FIG. 14B shows a situation where the passage is closed. The discharge adjusting valve 26 is screwed into a threaded hole formed at the base 1, has a semi-spherical end having the same diameter as the inner diameter of the passage, and is axially rotated to advance and retract from the discharge port 23, thereby providing a change in a passage sectional area of the discharge port 23. FIG. 14A shows a situation where the passage sectional area is about ½, and FIG. 14B shows a situation where the passage sectional area is completely closed. While operating the pump, the passage sectional area is not closed. The packing 28 is provided to reliably prevent a leak.

[0087] As is clear from the above description, according to the present invention, a pump capable of discharging at a small flow rate of 10 cc/minute may be constructed in a piston type pump, whereby a small discharge pump suitable for use in a system which requires a stable and small amount of fluid supply such as an oil supplying pump for a 2-cycle engine, a pump for supplying lubricant oil to a chain-saw, a small supply pump for medicine, for example, for intravenous drip. 

1. A piston type small discharge pump characterized in that a spring for biasing a piston towards a top dead center is provided outside a cylinder.
 2. The piston type small discharge pump as claimed in claim 1, wherein a flat type check valve is disposed in the vicinity of the bottom dead center of the piston so as to reduce a dead volume at the bottom dead center.
 3. The piston type small discharge pump as claimed in claim 1, wherein there are provided an internal recycling passage where an outlet side of a check valve disposed at an outlet side of the cylinder is communicated with an inlet portion side of a check valve at an inlet portion side of the cylinder, and a flow rate adjusting valve at said internal recycling passage.
 4. The piston type small discharge pump as claimed in claim 3, wherein there is provided a flow rate adjusting valve at a discharge port side.
 5. The piston type small discharge pump as claimed in claim 4, wherein a check valve is provided at said internal recycling passage.
 6. The piston type small discharge pump as claimed in claim 1, wherein there is provided either or both an outer diameter reduced portion where the outer diameter is reduced at a lower portion of the piston and an inner diameter increased portion where the inner diameter is increased at a lower portion of the cylinder.
 7. The piston type small discharge pump as claimed in any one of claims 1 to 6, wherein an envelope of a cam portion, mounted in the middle of a rotary shaft, for driving a piston is defined not to extend beyond an outer shape of the rotary shaft.
 8. The piston type small discharge pump as claimed in claim 7, wherein an intermediate member which substantially does not move as the cam portion rotates is interposed between the cam portion and a top of the piston driven by the cam portion.
 9. The piston type small discharge pump as claimed in claim 8, wherein the cam portion is column-shaped and eccentric of the central axis of the rotary shaft. 